Sims 3 freezing and crashing

I recently installed Pets and Generations using the ultimate fix. I could get the game started, but it gave me a message about "unauthorized modifications." Sometimes I could get as far as loading a household and playing, but sometimes the game would freeze while it was still showing me the loading screen, and I would have to alt-tab out, where I would find a Windows message saying the game had stopped working. Even when I did manage to play, I couldn't use Buy mode -- the game would just load and not show me anything.

I suspected this was related to the new update, so I uninstalled both Pets and Generations and hoped to continue playing as before. (I paid for the base game.) I restarted my computer and whatnot. But I'm still having problems with the game freezing while loading up and I'm not sure what to do to fix it. Do I need to completely uninstall the game and re-download and start all over?

Any advice would be greatly appreciated! Thanks!